1. Field of the Invention
The present invention relates to a process for producing microcapsules which use an aminoaldehyde polycondensate as a capsule-wall for the purpose of changing or controlling various properties of substances in such field as pharmaceuticals, agricultural chemicals, perfumes, liquid crystals, pressure-sensitive recording papers, heat-sensitive recording papers, and the like. More specifically, the present invention relates to a process for producing microcapsules which have an aminoaldehyde polycondensate wall effective for pressure-sensitive recording sheets.
2. Prior Art
The process for the present invention belongs to an in situ polymerization process, a chemical process in which a capsule wall composed of an aminoaldehyde polycondensate is formed by a reaction starting from the continuous phase.
The use of amine/aldehyde polycondensate or urea/aldehyde polycondensate for forming the wall film has already been disclosed in, for example, Japanese Patent Publication Nos. 37-12380, 44-3459, 47-23165 and the like. This process has the defects that the emulsification or dispersion is not efficiently achieved, or that it is difficult to efficiently and stably deposit the polycondensate around the hydrophobic core material.
In order to improve the above-mentioned defects, the use of acrylic acid polymers or copolymers as an anionic polyelectrolyte is disclosed in Japanese Patent Publication 54-16949, Japanese Patent Laid-Open Application Nos. 58-14942, 59-142836, 60-28819, 60-68045, 60-190227, 60-216838, 60-238140, 61-11138, 61-17491, 62-19238, 62-57645, 62-97638, 62-250943 and 63-134084. The processes of the above disclosures have the following problems requiring some improvements. Although the process of the Japanese Patent Publication 54-16949 improves the depositing efficiency of the polycondensate around the core material, it has the defects that the resultant capsule slurry exhibits a very high viscosity.
Japanese Patent Laid-Open Application Nos. 58-14942, 59-142836 and 60-68045 disclose a capsule slurry having a high-concentration, a low viscosity and a good particle size distribution. However, they provide as a defect a capsule slurry having an undesirable high viscosity at a higher slurry concentration.
Japanese Patent Laid-Open Application Nos. 60-190227, 60-216838, 60-238140, 61-11138, 61-17491, 62-19238, 62-57645, 62-97638, 62-250943 and 63-134084 provide a capsule slurry having a higher concentration and a lower viscosity in the use of the described anionic polyelectrolyte, but they provide as a defect a worse particle-size distribution. In using the capsules obtained by these conventional processes for pressure-sensitive recording sheets, the higher concentration and lower viscosity described above cause an economical advantage due to the high-speed coating, but the worse particle-size distribution has the defects of causing a smudging in the color-information and further a spot-like smudging on the surface of recording sheet. Furthermore, in a single-type pressure-sensitive recording sheet, a self-contained paper, a spot-like smudging appears remarkably.